Many manufacturers claim that their receivers are GNSS receivers. Some demonstrate this by showing how they track and use other-than-GPS signals. Others additionally demonstrate the user benefit an extra GNSS brings (usually position accuracy, availability, reliability).
And everyone concludes that GLONASS, Galileo, or Compass is (or at least can be) a good mate for GPS. The keyword here is “mate.” A mate is not a master; a mate is not an equal partner. A mate is someone who help, but brings no value by itself.
One may ask if a GNSS receiver can still work well without receiving GPS or, in other words, whether the GNSS receiver is GPS-centric or not. A GPS-centric architecture works only if GPS is available; there is no position solution without GPS.
The first commercial professional GNSS (GPS+GLONASS™) product appeared in 1996. Later, the world's first GPS+GLONASS RTK technology was announced and implemented. At that time, the GLONASS constellation was large, but a few years later it declined dramatically.
After years of illness, GLONASS has returned to the GNSS market place and become the strong second player in most professional receivers. The current constellation is almost complete, and full GLONASS deployment was expected in 2011. Moreover, some (few) manufacturers now support the GLONASS-only mode. And some others start to better appreciate the value of GLONASS having now a clear vision of the planned GLONASS extension with CDMA signals.
GLONASS aside, new GNSS satellites started orbiting in the sky. Until now, none of these GNSS systems yet brings any extra value, the only exception being SBAS ranging which however has limited applicability.
It means that today the observables of new GNSSs are usually not used in the determination of the receiver position. Just because there are only a few of them, they cannot add value to GPS and GLONASS observables.